The present invention relates to radio-frequency (RF) dielectric heating or drying; more specifically, the present invention relates to an improved system for coupling the RF power source to the applicator that allows improved electric field special uniformity and significantly reduced risks of catastrophic arcing failures.
In the present day application of radio RF power to a typical applicator (otherwise often referred to as the electrode or capacitance plate) used in dielectric heating applications, the RF generator is connected to the applicator by the well-known method of xe2x80x9cDirect Couplingxe2x80x9d. In xe2x80x9cDirect Couplingxe2x80x9d, the RF power is connected directly to the applicator and circulating currents (properties of generating electric fields) travel back from the RF applicator through the feed lines (including any feedthroughs), and back to the output sections of the RF generator or optionally a matching network (if a matching network is being used). The feedthroughs are the location where the incoming RF power feed lines pass into the heating system housing or the like.
Because of the inherent inductance of the RF feed lines and feedthroughs between the RF generator/matching network and applicator, operating at higher RF power levels produces higher circulating currents that often result in very high voltages to be generated on the RF feed lines, at the feedthroughs, and back to the output sections of the RF generator/matching network in direct-coupled applications.
With higher RF voltages on the feed lines, at the feedthroughs, and at the output sections of the RF generator/matching network (which can exceed 10 kV in typical dielectric heating applications), there are increasing risks of catastrophic arcing failure. With extremely high RF voltages (in excess of 50 kV), catastrophic failure is typically imminent in dielectric heating applications. In addition to the risk of calasioplic failures, it is often difficult/impossible or very expensive to find/design RF components that can withstand very high RF voltages in the feedthroughs, feed lines, and the output sections of the RF generator/matching networks. In direct-coupled applications where the RF voltage can become extremely high, the only reasonable solution to prevent catastrophic failure is to reduce the RF power output. However reducing RF power output also reduces process throughputs of the heating/drying system, which is often unacceptable to the process operator. The above-described problems have often resulted in RF power being perceived as not suitable for many otherwise suitable applications.
In a special application of RF power used in high-energy physics particle accelerators, an alternative method of coupling called xe2x80x9cInductive Couplingxe2x80x9d is known to be used for the sole application of generating electric fields to accelerate particles such as protons and electrons. xe2x80x9cInductive Couplingxe2x80x9d as employed in particle accelerators incorporates distributed inductance in resonance with the applicator strictly to reduce feed line voltages and create the appropriate resonant frequency but not to shape the electric fields. In these applications, the RF power is transferred to the applicator using the well-known principle of mutual coupling where the magnetic field established (by the feed line(s)) induces a voltage on the applicator. Furthermore to Applicant""s knowledge, inductive coupling as described above has never been applied to systems for dielectrically heating or drying materials in the electric fields.
With xe2x80x9cInductive Couplingxe2x80x9d, the circulating current path changes significantly from xe2x80x9cDirect Couplingxe2x80x9d; there is significantly less circulating current flowing in the feed line(s) directly connected to the applicator and very significant circulating current flows are created from the applicator through the distributed inductance section to ground potential. A benefit of this arrangement found by the Inventors and described below is a reduction in circulating current flow drastically reducing the voltages seen on the feed lines, feedthroughs, and output sections of the RF generator/matching networks.
With inductive coupling in particle accelerators, the RF applicator surface is typically circular and very small (less than 30 cm in circumference). In some cases, the applicator can be much longer but is generally less than 5 cm wide. In all cases, the inductively coupled RF applicators are non-movable, much too small to be suited for more industrial dielectric heating applications, and designed specifically for accelerating particles.
Notwithstanding these perceived limitations, the present invention presents a novel approach to expand xe2x80x9cInductive Couplingxe2x80x9d into dielectric heating applications.
It is an object of this invention to provide an improved RF heating or drying system.
It is a further object of the invention to provide a method and apparatus for RF heating or drying incorporating inductive coupling.
It is yet another object of the invention to provide a flexible electrical connector for connecting an applicator to an RF source in an RF heating system.
Broadly the present invention relates to a method and apparatus for heating or drying material by applying radio frequency (RF) power to said material in a resonant cavity; the improvement comprising inductive coupling an RF power source to said resonant cavity formed by at least one feed line delivering said RF power, a distributed inductance in resonance with an applicator, said applicator and said material and generating a magnetic field that induces a voltage on said applicator permitting voltages on said feed line(s) delivering said RF power to said cavity to be lower than those that would normally be encountered for equivalent RF heating using direct coupling.
Preferably said generating a magnetic field comprises using said distributed inductance to form a conducting loop with said feed line(s).
Preferably said distributed inductance shapes the electric field within said cavity to provide a uniform electric field intensity applied to said material.
Broadly, the present invention relates to a radio frequency heating system comprising a grounded conductive chamber an applicator in said chamber, said applicator including conductive electrodes, means connecting said applicator to a source of radio frequency power and a distributed inductance means connecting said applicator to the chamber.
Preferably, said chamber comprises a grounded conductive box having a pair of opposed side walls and a bottom and a top wall, said applicator extending laterally of said box, between said side walls, and said distributed inductance means connecting said applicator to its adjacent of said side walls.
Preferably said distributed inductance means comprises a pair of distributed inductance sections one of said distributed inductances sections connecting one side of said applicator to the adjacent side of the chamber and another of said pair of distributed inductance sections connecting a side of said applicator remote from said one side to the adjacent side of the chamber.
Preferably each of said inductance sections has a first portion connected to its end of said applicator, a second portion connecting said first portion to a third portion which is connected to its adjacent said side walls. Preferably, said applicator is hollow and may have perforations for hot air connecting a surface of said applicator facing said material to a hollow interior of said applicator.
A flexible feed line for connecting radio frequency power from a feedthrough to an applicator, said feed line comprising a plurality of wire bundles woven together to form a hollow cylindrical braid connector having an outer surface, more than 20% of the area of said surface being formed by said wires and less than 80% of said surface by air, said air and wire areas being symmetrically uniformly positioned over said surface and collectively establishing a known inductance. The maximum amount of surface area occupied by the wires may approach 100% depending of the flexibility required of the connector, which is dependent on the flexibility and the fineness of the wires
Preferably each said bundle comprises between 3 and 10 wires in side by side relationship.
Preferably said hollow cylindrical braid has an elliptical cross section.